The basic components of an RFID chip or tag for an RFID card comprise a substrate (usually a plastic film or paper), a patterned conductive antenna and an integrated circuit which may store data and be interrogated. The RFID industry is mainly driven by cost and significant advances have been made in reducing the cost of the chips which are used in RFID. A significant amount of work has also occurred in bringing down the cost of the antenna. The utilisation of additive metallization methods, printing and low-cost materials have all contributed to a reduction in the cost of antenna production. However, a significant cost in the manufacture of the RFID card still comes from the process of attaching the chip to the antenna. As well as the need to form a conductive bond between the chip and antenna, the precision with which the chip must be placed requires expensive and relatively slow equipment.
RFID antennas and other electrical conductive elements are traditionally manufactured by a process of masking and etching metal-covered substrates. For example, a plastic or composite substrate (e.g. PET or FR4 epoxy composite) is co-laminated with a copper or aluminium foil. The metal structure would then be coated and patterned with an etch resistant material (e.g. photoresist) and then immersed in a chemical etchant (ferric chloride, nitric acid or a mixture of peroxide and sulphuric acid in the case of copper, hydrochloric acid or sodium hydroxide in the case of aluminium) to remove the excess metal foil. The etch resist would then be removed to leave a patterned metal layer in the form of an antenna. The RFID inlay would then be completed by attaching an RFID chip to the antenna such that raised contact bumps on the surface of the chip come into electrical contact with contact pads at the feed points of the antenna. This may be done by first connecting these bumps to a larger metal structure (known as a strap or interposer) which is then bonded to the antenna, or by directly placing the chip face down on the antenna with the contact bumps on top of the contact pads. In either case, good electrical contact and mechanical bonding are achieved by the use of a conductive adhesive paste or film. In many cases this adhesive is anisotropically conductive (i.e. the conductivity in the lateral or in-plane direction is much lower than that in the vertical direction) so as to prevent shorting between the bumps.
In an alternative approach the antenna itself may be fabricated using an additive patterning technique. Under this methodology the conductive region is added patternwise (usually using a printing technique) to the insulating substrate. This may be done by printing a paste or suspension of metal particles such as silver or copper. Alternatively it may be achieved by printing a catalytic material which is then coated with metal using a process of electroless deposition. In either case the antenna conductivity may be further increased by subsequent electrolytic plating.
In the above cases, the conducting materials in their chosen form are usually found to be far more costly than simple metal foils. In particular, aluminium foil is particularly abundant and available at significantly lower cost (per unit area) than the materials usually used for antenna manufacture. Consequently, some manufacturers choose to directly die-cut aluminium foil to form antennas. This method, although cheap, greatly limits the complexity of the antennas which may be formed and leads to great difficulties in the handling of the materials during subsequent processes.
In addition to the cost of the antenna, the cost of accurately locating the bumped pads of the chips to the contacts on the antenna is also significant (in terms of process time and the capital equipment involved).
As an alternative approach, the applicants have demonstrated (e.g. in WO 2005/044451) the ability to place the chip relatively inaccurately with the contact pads facing upwards, and utilise the digital nature of inkjet printing to print the connection between the antenna and the contact bumps. Even so, this process still requires the fabrication of an antenna by either a traditional additive or subtractive method. The invention employs a different way of ensuring that there is accurate registration between the component and the electrical circuit to which it is to be electrically connected.